Magnetrons are well-known vacuum electron discharge devices used to generate electromagnetic filed in the microwave frequency range. A typical magnetron comprises a cylindrical cathode, a cylindrical anode surrounding the cathode and a plurality of resonant cavities formed in the anode by either slots or vanes. An electric field is established between the cathode and the anode, and a magnetic field is applied perpendicular to the electric field in the so-called interaction region, which is the evacuated space between the cathode and the resonant cavities in the anode. When the value of E/H is suitable, electrons emitted from the cathode interact with the electric and magnetic fields to generate microwave energy at a frequency determined by the parameters and the resonance characteristics of the cavities.
A known problem with magnetrons is that of unwanted emissions. These generally take two forms: the first is amplification of a harmonic of the desired mode. The other form of unwanted emission is called moding; that is, significant cavity responses occurring at frequencies other than the frequency for which the magnetron is designed to operate.
Conventionally, harmonic emissions are reduced by means of external microwave filters or by employing complex output systems embodying integral filters.
The concept of strapping was devised as a solution to the problem of moding. Strapping, as conventionally understood, is the joining of alternate anode vanes by means of a closed ring. Each pair of vanes forms a resonant cavity there between, and all such cavities are maintained at the same electrical potential. Thus, alternate anode vanes are locked into operation in the desired mode, known as the π mode. The π mode has node points of the same distance apart as the distance between alternate vanes.
A problem has been encountered with conventional straps in that the currents that circulate in the straps, to equalise the voltage on alternate vanes, may also generate an electromagnetic field. This field may interfere with the operation and performance of the magnetron owing to its proximity to the cathode.
A proposed solution to this problem is that of providing a break in the strap. This prohibits the flow of current between the vanes that the strap connects. However, a drawback of this approach is that it increases the frequency separation of the modes of operation and may also cause oscillations at undesired frequencies.